1. Field of the Invention
The present invention relates to a relay for use in telecommunications equipment. It applies with particular advantage to telecommunications satellites.
2. Description of the Related Art
Telecommunications satellites are increasingly used in increasingly large numbers to broadcast content to end users. The content can relate to broadcast television channels, for example, or to Internet sessions.
However, the invention also finds applications in other kinds of telecommunications equipment, in particular in ground equipment.
FIG. 1 shows the use of a telecommunications satellite. It shows four ground stations S1, S2, S3 and S4, i.e. telecommunications equipment providing a link between one or more telecommunications satellites and a terrestrial telecommunications network (not shown). The ground stations can transmit data to a telecommunications satellite S and can receive data from it.
The data is structured in the form of information cells that take various forms, depending on the communication protocols used. A set of information cells sent by a ground station to a telecommunications satellite forms an uplink information stream. Conversely, a set of information cells sent by a telecommunications satellite to a ground station forms a downlink information stream.
Communication channels carry the uplink and downlink information streams and can be set up at the request of a ground station at the same as determining an associated bandwidth, i.e. specifying the likely volume of the information streams carried by the communication channels.
FIG. 1 shows two uplink information streams TD and TR and three downlink information streams TR1, TR2 and TD1.
Telecommunications satellites can also have an information stream duplication function. Thus the information stream TR is duplicated in the telecommunications satellite S and sent to the two ground stations S3 and S4 in the form of the two downlink information streams TR1 and TR2.
Two types of information stream pass through a telecommunications satellite: real-time information streams and differed-time information streams.
During an Internet session, multimedia information conforming to the hypertext transfer protocol (HTTP) is sent in real time, because the user wishes to experience the shortest possible time-delay between requesting a multimedia document and seeing it on the screen of their communication terminal. On the other hand, a film can be downloaded from a server to the user's terminal in differed time.
The uplink information stream TR and the downlink information streams TR1 and TR2 in FIG. 1 are real-time streams. The uplink information stream TD and the downlink information stream TD1 are differed-time streams.
The bit rates of the information streams may vary. Consequently, to avoid congestion, the satellite and the communication channels are generally designed for the maximum possible bit rate, or the permitted number of users is defined relative to the bit rate needed to convey the information streams simultaneously in real time.
For example, a real-time information stream is generally associated with a bandwidth reserved at the same time as setting up the communication channel to carry the information stream. As already indicated, at any given time the bit rate of the real-time stream is not necessarily the same as the reserved bandwidth.
For example, the real-time stream may correspond to a television channel. Television transmissions are generally coded differently and with different qualities; for example, films are coded with high quality that can use the whole of the reserved bandwidth, whereas other transmissions, requiring lower quality, use only part of the reserved bandwidth.
In this case, the unused part is filled with cells known as “filler” cells so that this underuse is transparent for the various equipment units of the system.
Consequently, it is clear that the reserved bandwidth is in practice rarely used, with the result that a significant proportion of the capacity of the resources employed is underused.
Thus the problem arises of underuse of the capacity of the resources of the communication satellites, in particular underuse of the downlink channels.
A first solution is to use the asynchronous transfer mode (ATM) technology to improve the use of resources.
However, this kind of solution is not satisfactory.
This is because the bit rates to be processed onboard the satellite require aerospace grade application-specific integrated circuits (ASIC), which have very limited memory capacities. Cells reaching the satellite are present only very briefly, as it were, and, to avoid memory congestion, it is essential to monitor the stream before it is sent by the ground station. Thus, using this solution, it is necessary to verify that the resource is available before sending any data packets.
This implies dedicated equipment on the ground, which is costly.